Starting and running issues

[IanCordes]

I agree with John. Zooming in on the photo shows the flange bolts are tightened right up, so the faces are touching. They are not touching in the centre, so I suspect overtightening, distorting the carb flange.That will affect starting. The face will need truing before re-fitting, then use plain nuts with spring washers, tightening just sufficient to flatten the washers, whilst leaving a gap between the faces you can get a 0.025" feeler gauge in.

Worse, though, is the other effect of distorting the flange is to distort the carb body. This will cause an interference with the slide, preventing the throttle from closing properly. There is not an easy fix for this, apart from a new carb body, although I do recall an article, possibly in an old LL, which did describe how it can be done.

I suggest you read this:- https://amalcarb.co.uk/installing-amal-carburetters

[iansoady]

It's amazing how many people make this simple error. Like Ian I use spring washers under the nuts and only tighten till they're just flattened. I also use reduced hex nuts which allow you to get a spanner on them. The problem with flatting the flange is, as I an says, that the body will have distorted. I have had some success by putting the body (less slide) in the vice with pieces of wood either side of the flange (where the bolt holes are) and carefully squeezing - an attempt to reverse the original distorting force. It's by no means a perfect solution but better than flatting in my experience.

[IanCordes]

It would certainly be best to attempt to eliminate any body distortion before flatting the flange face, which may still need a tweak. The point about fitting the carb fully assembled, including the top, is often overlooked, as one might bolt the carb up first, then connect the cable and fit the top after; wrong procedure. This is an issue more prevalent with the Concentrics than Monoblocs, which had thicker flanges; although it can still happen.

[johnrunnacles]

Ian C may be onto something with his query whether it is a two-stroke carb. A lot of new Concentrics marked up as for 2-stroke fitting seem to have escaped from one reputable source with 4-stroke internals. First point of reference (without dismantling the carb) is the top of the spray-tube, which should have been chamfered for two-stroke compatibility.

[Peter E.M]

Thank you everyone for your help with this. I'll have a look at carb distortion. I wonder if it will be obvious to an new be like me, do you think the distortion would cause the slide to physically stick and and will it be obvious. Funnily enough I remember watching a Henry Cole programme where Sam his friend, mentioned not over tightening the flange bolt he said it was a common mistake people made.

[John Wakefield]

Thank you everyone for your help with this. I'll have a look at carb distortion. I wonder if it will be obvious to an new be like me, do you think the distortion would cause the slide to physically stick and and will it be obvious. Funnily enough I remember watching a Henry Cole programme where Sam his friend, mentioned not over tightening the flange bolt he said it was a common mistake people made.

Yes a common problem with ham fisted owners in the past. One thing you could do (without removing carb) to see if there is an air leak on flange. Get engine running at low speed and spray some carb cleaner or WD40 around the flange, if engine speed then increases that will indicate an air leak.

[AndyB]

Yes a common problem with ham fisted owners in the past.

To be honest, there are not many fasteners on an old British bike that shouldn't be done up good and tight!

[Peter E.M]

I try the WD 40 test tonight

[IanCordes]

If the body is distorted, the slide will be sticky in the bore, maybe not all the way up, but it only needs to stick at one point! Check the flange face on a flat surface for distortion. It certainly looks distorted in the photo. Also, check that the spray tube has a chamfer on one side. The needle and needle jets differ from 2-stroke to 4-stroke. Visually the 4-stroke needle jet has cross drillings on 2 flats of the hexagon, the 2-stroke does not.

[Peter E.M]

Ok I'll check all of that. Thank you

[Peter E.M]

Well yesterday I got to having a look at the carb. I flattened the carb face and the manifold face as well. The carb was a little "unflat" . I checked the slide and it seems ok. I put it together and it started after a few kicks and tickles. I'm wondering how fast you have to get the engine spinning for the electronic ignition to work correctly, I've had a bad back so kicking it over has not been easy so maybe sorting the carb flange out and giving it a healthier kick have both helped. It still doesn't seem to shut down smoothly after it has been revved up, is this how they run anyway? I think the carb is the correct one for a 2-stroke. When you look at the tube where it comes into the Ventura it does have a cut away or chamfer on the side facing the engine